Black Sea dissolved organic matter dynamics: Insights from optical analyses

Abstract

To obtain qualitative information on the Black Sea's dissolved organic matter (DOM) pool, the optical properties (absorption and fluorescence) were measured in 111 samples collected across the basin. Chromophoric dissolved organic matter (CDOM) was considered at three wavelengths (254 nm, 280 nm, and 325 nm), along with the spectral slope between 275 nm and 295 nm (S275–295) and the specific ultraviolet absorbance at 254 nm (SUVA254). Using parallel factor analysis, a five-component model identified three humic-like components, a protein-like component, and a polycyclic aromatic hydrocarbon-like component. In the basin's oxic layer (upper ∼ 100 m), protein-like CDOM was elevated, likely due to the production of this labile material, while humic-like material was low, suggesting its removal by photo-oxidation. In the underlying waters, the protein-like material decreased, perhaps due to the utilization of this nitrogen-containing DOM, while humic-like material increased, suggesting its production at depth. In the anoxic layer (lower ∼ 2000 m), dissolved organic carbon (DOC) varied by only ∼ 10% while CDOM increased with depth by a factor of approximately two; the optical properties correlated well with the H2S-equivalence of mineralization, referred to here as apparent carbon mineralization (ACM), while DOC did not. The strong correlation between CDOM and ACM is similar to correlations previously identified in the open ocean that compared CDOM with apparent oxygen utilization, suggesting that CDOM accumulates as a function of mineralization, independently of the oxidizing agent (i.e., oxygen or sulfate).